Tires having treads with central circumferential grooves for improved wet traction are very well known in the prior art. Goodyear's AQUATREAD tire is one such prior art tire, and is the subject of the following patents and patent applications: U.S. Pat. Nos. 4,700,762 and 5,176,766, and European Patent Publication Nos. 503 404 A1, 503 405 A1, 503 406 A1 and 503 407 A1. The AQUATREAD tire and corresponding prior art publications all appear to disclose a tread including a "depression" or "aqua channel" at the mid-circumferential centerplane of the tire and a plurality of unblocked lateral grooves which merge with the depression to channel water to the tread edge when the tire is rotated in the sole, proper direction.
The '762 and '766 patents, and the '404 and '407 publications generally disclose the depression or aqua channel as having a depth of about 78-100% of tread depth. One disadvantage of the AQUATREAD tire is that the aqua channel may have diminished effectiveness as the tread wears and indeed may be detrimental to wet traction since channel depth may be exhausted with 22% of tread depth remaining on the tire. Another disadvantage is that because the AQUATREAD tire is designed for unidirectional rotation, it may not be effectively used on both sides of a vehicle without being removed from and remounted on the wheel or hub.
U.S. Pat. No. 3,830,273 discloses a broad central, circumferential groove to improve wet traction for very wide tread (large contact area) tires conventionally used to improve curve holding and dry traction on very fast vehicles. The broad central, circumferential groove essentially permits a single tire to replace twin tires. The groove disadvantageously requires a completely reengineered carcass which includes a wire reinforcing rod of diameter intermediate between the bead diameter and the tread diameter. Another disadvantage is the further complexity and expense of providing independent tread reinforcements for each of the tread portions.
In U.S. Pat. No. 3,789,898, central groove 15 includes a cut 16 to provide an articulation joint to the tire tread, so that the tire, preferably a truck or bus tire, moves more easily over any difference in level between the road and shoulder. The articulation joint reduces the need to move the whole tread to a single surface for vehicle control and to reduce swerving. Cut 16 has negligible width and apparently would not be effective to improve wet traction. A further disadvantage is that the provision of separate belt plies on either side of cut 16 increases the complexity and cost of manufacturing the tire.
U.S. Pat. Nos. 4,289,182 4,785,863 and 4,796,683 all appear to disclose circumferential tread rubber ribs, however, these ribs do not reinforce central circumferential grooves deeper than tread element height and do not urge water into the subtread portion of the central groove. In the '863 patent, the ribs separate portions of the tread designed for wet and dry traction.
What is desired, therefore, is a tire having a tread which provides improved wet traction throughout tread element wear without necessitating either a supplemental reinforcing wire, a distorted carcass, or a dual reinforcing belt system. A tire having improved wet traction when the tread rubber is worn completely off is also desired.